Facile and stereoselective synthesis of (Z)-15-octadecenoic acid and (Z)-16-nonadecenoic acid: monounsaturated omega-3 fatty acids.

Facile syntheses of the monounsaturated omega-3 fatty acids, (Z)-15-octadecenoic acid and (Z)-16-nonadecenoic acid, are presented. Commercially available hydroxy fatty acids were esterified and oxidised, followed by the Wittig reaction to introduce the omega-3 olefinic bond; hydrolysis yielded the omega-3 fatty acids in high purity. An examination of different reaction conditions for the Wittig step found that THF as solvent and coupling temperatures of -78 degrees C gave optimal stereoselectivity, affording the omega-3 olefins in Z:E ratios >or= 97:3. The syntheses have overall yields of approximately 43%, and utilise straightforward, robust chemistry, that may be readily scaled up and reproduced. Also presented is a method for accurately determining the double bond geometry and isomeric purity of the fatty acid products using 1H-13C-HSQC NMR and GC-MS, respectively.